For example, as a developing unit, JP-A-9-319202 discloses a related developing device, in which a developer cartridge accommodating developer is removably mounted to a frame. In the related developing device, a hole formed in one side portion of the developer cartridge and a hole formed in the frame form a developer supply opening, and a hole formed in the other side portion of the developer cartridge and a hole formed in the frame form a developer receiving opening.
The developer accommodated in the developer cartridge is supplied to the frame through the developer supply opening, but a part of the developer is returned to the developer cartridge through the developer receiving opening. Accordingly, the developer is circulated between the frame and the developer cartridge.
The developer cartridge has a double structure of the first frame accommodating developer and the second frame that is provided outside the first frame.